A prior art system of the type referred to is arranged as shown in FIG. 11.
A base unit 1, which is coupled through a radio communication circuit to a wireless telephone set 2, is also connected to a wire telephone line 3.
The outline of the operation of the base unit 1 will be explained in the following.
A signal transmitted from the wire telephone line 3 is applied through a line relay 33 and a hybrid circuit 4 to a transmitter 5 to be modulated thereat and to obtain modulated radio waves. The modulated radio waves are sent through an antenna duplexer 6 to an antenna 7 to be transmitted from the antenna to the radio telephone set 2.
Radio waves transmitted from the radio telephone set 2, on the other hand, are received at the antenna 7 and then sent through the antenna duplexer 6 to a receiver 8 to be demodulated therein and to obtain a demodulated signal. The demodulated signal is sent onto the wire telephone line 3 through the hybrid circuit 4 and the line relay 33.
A synthesizer 9 outputs a frequency corresponding to a radio channel to the transmitter 5 and the receiver 8.
One output of the receiver 8 is applied to a received-electric-field detecting circuit 10 to determine the intensity of the received electric field. This circuit 10 is usually called a carrier squelch circuit or a noise squelch circuit.
The other output of the receiver 8 is output as a data signal included in the received demodulated waves, that is, it is applied to an identification signal detecting circuit 11 to be compared with an identification signal that is determined by a combination of the base unit 1 and the radio telephone set 2. This identification signal is generally called an ID code.
An output of the detection circuit 11 and the demodulated output of the data signal of the receiver 8 are input to a control circuit 12 to be used for control of interconnection. The control circuit 12 in turn controls the synthesizer 9 for control of the radio channel, and also controls to cause the supply of a transmission data signal to the transmitter as a modulation input.
Similarly to the base unit 1, the radio telephone set 2 also has an antenna 13, an antenna duplexer 18 and a receiver 14, in which a demodulated output of the receiver 14 is sent to a telephone receiver 15. A voice input signal from a telephone transmitter 16 is applied to a transmitter 17 to be modulated and then sent through the antenna duplexer 18 to the antenna 13, from which the voice signal is radiated in the form of radio waves.
A synthesizer 19, a received-electric-field detecting circuit 20 and an identification signal detecting circuit 21 are the same as those in the base unit 1, and a control circuit 22 performs control over the radio telephone set 2. A loudspeaker 23 is a sounder which emits a calling voice indicative of a receiving call.
With respect to the power of these circuits, the base unit 1 is connected through an AC plug 24 to a commercial AC 100 V plug receptacle so that outputs of a rectifier and stabilizer circuit 25 are distributed to the respective circuits of the base unit. One of the outputs of the circuit 25 is also supplied through a current control resistor 26, charging terminals 27 and 28 to a chargeable battery 29 to use the output of the battery 29 as the power of the radio telephone set 2.
The control of the above prior art example in a signal reception mode will be schematically shown in FIG. 12.
More specifically, when the base unit 1 detects a ringing signal from the wire telephone line 3 at a ringing-signal detector 30 in an await mode (step 34), the unit 1 sets the synthesizer 9 for the control channel and turns ON the transmitter 5 to transmit the received signal (step 35). The received signal includes a signal specifying a speech channel (S-CH).
The radio telephone set 2, on the other hand, turns ON the synthesizer 19 for a period of a predetermined time t1 in an await mode to set the synthesizer for the control channel and also turns ON the transmitter 14 (step 36). Under such a condition, if the radio telephone set 2 receives the received signal (step 37), then it turns ON the transmitter 17 (step 38), transmits an answer signal including the ID code in response to the receive signal (step 39), and switches the speech channel (S-CH) to the specified one (step 45). When the radio telephone set 2 receives no receive signal, it turns OFF the synthesizer 19 and the receiver 14 for a period of a predetermined time t2 (step 40). The intermittent signal reception of the receiver 14 based on its ON and OFF operations is called battery saving.
More in detail, assuming that the receiver 14 in its OFF state consumes a current I OFF and the receiver 14 in its ON state consumes a current I ON, then an average consumption current IA in the await mode of the radio telephone set 2 is expressed as follows. EQU IA=(t1.times.ION+t2.times.IOFF)/(t1+t2)
Since a relation of I ON&gt;I OFF is usually satisfied, the average consumption current IA can be made small.
The base unit 1, when detecting radio waves transmitted from the radio telephone set 2 at the received-electric-field detector 10 (step 41), stops the transmission of the received signal (step 42). At this stage, if the field detector 10 detects no radio waves, then the base unit 1 continually transmits the received signal up to a predetermined number of times n (step 43). The predetermined-time transmission is due to the intermittent reception of the radio telephone set 2. That is, the radio telephone set 2 cannot receive the transmitted signal for a time period of t2, and thus the transmission is carried out preferably for a period of a time longer than the time t2 by an amount corresponding to 2 received signals. The abortion of the transmission up to n times is due to the avoidance of the useless occupation of the control channel in the event where the radio telephone set 2 is powered off or located a long distance from the base unit 1.
Next, when the ID code included in the received-signal answer signal from the radio telephone set 2 coincides with the ID code set for the base unit 1 (step 44), the base unit 1 switches the speech channel S-CH to the one specified by the received signal. If the two ID codes do not coincide, then the received answer signal is the one from another radio telephone set and thus the base unit waits until the ringing from the wire telephone line 3 is terminated (step 47) and returns to its await state.
After switching to the speech channel, the base unit 1 transmits a bell ringing signal (step 47). The radio telephone set 2, when receiving the bell ringing signal (step 48), controls the sounder 23 to generate a calling sound (step 49). Under this condition, an off-hook operation of the radio telephone set 2 (step 50) causes the telephone set 2 to transmit an off-hook signal (step 51), thus putting the telephone set in its speech mode (step 55).
On the other hand, the base unit 1, when receiving the off-hook signal (step 52), stops the transmission of the bell ringing signal (step 53), and closes the line relay 33 to establish a speech loop with the wire telephone line 3 and enter its speech mode (step 54).
When the number of transmission times at the transmitter reaches n, the base unit detects the termination of the signal reception (step 56) and returns to its await mode, thereby preventing the unnecessary signal-reception re-operation.
In this way, it is ensured to establish a radio telephone set loop only with a party having a coincided ID code, thereby preventing any erroneous interconnection and telephone charges resulting from radio interference. The respective ID codes are previously written in associated ID ROM's 58 and 59, and the ID ROM's are both of an electrically non-volatile type to avoid the extinguishment of such ID codes even in case of a power failure.
Such a system as mentioned above has had a problem that since the base unit 1 and the radio telephone set 2 have respectively ID codes fixedly written in the associated ID ROM's, when it is desired to modify the combination therebetween or add a new combination, it is necessary to open the respective casings of the unit 1 and telephone set 2 and to exchange the previously written ID ROM's or add new ones. For example, when it is desired to use a plurality of radio telephone sets with respect to the single base unit 1 or to move a radio telephone set to another place and to establish a radio circuit between the telephone and another base unit, it has been impossible for the prior art system to realize it.
In view of such circumstances, it is an object of the present invention to provide a radio telephone system which eliminates the above problem in the prior art and which allows the easy addition and modification of combinations between radio telephone sets and base units.